Voltage or gain control



`Fan. 25, 1938.

F. B. ANDERSON ET Al. 2,106,336

VOLTAGE OR GAIN CONTROL Filed Oct. 8, 1935 FIG.

DEPARTURE FROM LINEAR/TY 0F GAIN CHANGE (DE) TOTAL CAPACITY A7' ZERO SETTING 0F C,

DIAL SETTING 0F C] F. B. ANDERSON INI/E N TORS C. 0. MALL INC/(RODT A.L STILLWELL HVJ.

A 7" TOR/VE Y Patented Jan. 25, 1938 VOLTAGE OR GAIN CONTROL Frithiof B. Anderson, Bloomield, N. J., Charles O. Mallinckrodt, Forest Hills, N. Y., and Albert L. Stllwell, Westfield, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation o1 New York Application october s, 1935, seriaiNo. 44,050

4 Claims.

t l The present invention relates to a method and means for facilitating deriving from a given volt age desired fixed or variable portions of the 1 voltage.

A feature of the invention comprises separate means of variation between a point of application of a voltage to a circuit and a point at which a desired portion of the voltage is to be taken 1 off, such that these means are enabled to -vary l: the voltage taken off, separately andl independently of one another.

More specically, the invention comprises use of'capacities in place of resistance potentiometers, with the rate ofV variation of capacity made '15 to eiect desired rate of change of voltage, and

with this rate of change unaffected by other capacity means in the circuit for separately varying the voltage.

The invention is of general application but 20 will be illustrated and described as -embodied in a gain control system which is specifically disclosed and claimed in a patent of F. A. Brooks, No. 2,075,975 issued April 6, 1937. In that patent an amplifier is disclosed as having its gain 25 controlled by means of a variable condenser .in

the gain-reducing feed-back circuit of the ampliiier. 'Ihe changes in gain thus produced are made to compensate for variable transmission characteristic of the system by suitable auto- 30 matic control. An additional gain control is also provided in the form of variable capacity in the gain-reducing feed-back path, and it is desirable that the gain changes effected by this latter control should be independent of those 35 produced by the first-mentioned control means. The second control may be used, for example, to adjust to the normal gain at the time of installation, after which it is left fixed.

The nature of the invention and its various 40 objects and features will be more fully underistood from the following detailed description taken in conjunction with the drawing in which:

Fig. 1 is a simplied schematic diagram vof the circuit of a line containing a repeater adapted 45 to have its gain regulated in accordance with this invention;

Fig. 1A is a detail modification of the portion of the circuit of Fig. 1 between the lines X-X vand Y-Y;

Fig. 2 shows graphs to be referred to in the description; and

Y vFig. 3 shows the shape of condenser plate used by applicants in practicing one form of the invention.

In Fig. 1, the line IIJ may be any transmission line such as the multiplex carrier transmission line of the Brooks patent, having inserted at a point along itthe .repeater circuit comprising amplifier Il. This amplier is'provided with a feed-back circuit shown as-.including attenuation equalizer I2 and gain changing condensers I3, I4, I5, and I6. The purpose of the feed-back circuitsis primarily to feed back from the output tothe inputr of the amplier a portion of the output waves in a manner to' reduce the gain of the amplifier,'the improvements in stability of the amplification and reduction in modulation being substantially proportionalto the gain reduction lby feed-back as disclosed by H. Y S. Black in his application'` Serial No. 606,871, filed April 22, 1932. Equalizer vI2 vhas a char: .acteristic similar to thatof the line IIJ and in'- fluences the feed-back waves in such manner as to produce a gain-frequency curve for the repeater such as to compensate for unequal attenuation of the line over the utilized frequency range, for example 12 to 60 kilocycles. l

'I'he attenuationofgline VIl lin practice varies with temperature or vother conditions and it is 'necessary or desirable to change the repeater gain to compensate suchline'jchanges. This may be done by varying the capacity (C1) of con- .denser l by any suitable control means such as a pilot wire regulator as shown by Brooks. If the degree` of feedfback is larga-under certain circuit conditions ton be pointed out below, changes in C'1 produce like change in gain over the entireyfrequency band. The rate or law of variation of C1 may be whatever the -designer wishes to makev it.. ,Y I' l` 'y The gain of the repeater can also bev ,changed by varying the capacity-@(Cz) of condenser I3 and the capacity (C3) of condenser I4, and if these ,tWocondensers `be vvaried in4 unison such #that Cfz-,l-Ca is a constant-triel gain change so produced is-independent of the gain-changes `Yef,- fected by varying C1. In installing the repeater, the gain setting for mean temperature of the line I0 may be adjusted to the proper value by changing both condensers .I3 and I4 Vtogether while keeping the sum of their capacities Carl-C3 constant. Then as the line attenuation changes, condenser I5 maybe changed by anautomatic pilot control or otherwise and the gain changes will be of proper valueand Isign' ifthe'operation of the control mechanism is properly related to the line attenuation changes fof the line, even though indifferent repeater locations inv which the repeater may need vto be installed different settings of the condensersv I3 and I4, as de- 55 scribed, may be necessary to give the repeater its normal gain. Thus, the same design of pilot control mechanism can be used at all repeater stations even though the repeater sections may vary in length and require repeaters of diierent normal gain.

'I'hat this independence of the two gain controls exists may be shown inthe following manner: Where a high value of amplification factor ,u is used for amplifier I I, if indicates the feedback factor, the product islarge and the amplification may be made to approach numerically the value f Y Y as shown by Black. Where a large amount of feed-back is used approaching this condition, the.

change in amplifier gain is 'earlyequal to the change in the ratio lj i f jor .-[Halcal This last expression is in the form of a product of two factors. The lack of dependence of the factors on each other is clear if Cz-l-Ca is always kept equal to Ck as postulated. The fed back voltage can be varied by changing either factor, and hence by changing the value of or the value of C1. Moreover, the rate or law of change of the fed back voltage produced by Varying C1 is the same for various settings of (their sum being constant).

As stated, the plates of condenser I may be shaped sothat a given rate of angular movement under control of suitable control mechanism produces capacity changes suitable for compensating attenuation changes of a line. Fig. 3 shows a shape of plate used by applicants in an actual system.- The rotor and stator plates were both of the same contour. This same type A`of condenser and the same pilot wire mechanism was used at all repeater points on the line notwithstanding that some repeater sections were of greater attenuation than others, requiring diierent normal repeater gains.

By use of a trimming condenser I 6, the gain change characteristic of the regulating condenser I5 may be modified considerably thus adapting the same condenser design -to Various situations. If the total capacity Cs obtained by adding together C1 at its minimum setting, C2, C3 and the capacity of 16 equalled 130 pf in a particular circuit used by applicants, then as C1 above was varied from minimum to maximum, a 12 decibel linear change in amplifier gain was made as indicated by the middle curve of Fig. 2, which is a straight line.l As the capacity of I6 was changed tojproduce other values of total capacity Cs, the gain changes 'for variations of C1 followed different shaped vcurves as shown in this iigure. The trimming condenser I6 thus offers a convenient means of compensating for lack of correspondence between gain changes produced by C1 and the gain changes required for exact compensation for Variations in line attenuation. Condenser I6 may be set at a Value corresponding to one of the curves of Fig. 2 or some other desired curve and kept at that value so long as thecharacteristic of the rest of the system remains unchanged.

In Fig. 1A the two condensers I3 and I4 of- Fig. l have been replaced by a three-plate construction consisting of fixed plates 23 and 24 and movable plate 22 which movesso that as its capacity with respect to plate 23 decreases, its capacity with respect to plate 24 increases by a like amount. This construction may be used to facilitate keeping the sum Cz-l-Ca a constant.

Various A.departures may be made from the illustrative embodiment that has been given, and from the values stated, without departing from the spirit and scope of the invention as defined in the claims..

What is claimed is:

1. An aperiodic circuit for transmitting waves of a broad band of frequencies, having a pair of input terminals` across which a voltage comprising said broad band of frequencies is impressed, two variable capacities connected in series across said terminals, means for varying said capacities while keeping their sum constant, whereby a fraction only of said impressed voltage appears across one of said two capacities, separate. adjustable means to Vary the transmission loss through saidgcircut over said frequency band comprising a third Variable capacity connected across said one only of said two capacities, and an output utilization circuit of high impedance connected across said third capacity.

2. An amplier having a gain-reducing feedback circuit feeding back such a large voltage that the Vchange of gain of the amplier circuit as a Whole including the feed-back path is substantially proportional to the change of the ratio of the terminalvoltages of the feed-back path, one gain changing means, comprising a pair of capacities respectively in series and in bridge of the feed-back path with means to vary said capacities while keeping their sum constant, and a second gain changing means comprising a variable capacity in bridge of said feed-back path in parallel withv one of said pair of capacities.

3. In a gain regulating circuit for a repeater adapted for; insertion in a transmission line, one variable capacity type gain changing means for setting the repeater gain to its required normal value, another variable capacity type gain changing means operating independently of the rst gain changing means for adjusting the gain of said repeater to other Values in accordance with a predetermined relationship.

4. A repeater adapted for use in connection with line sections of different attenuation, comprising one gain changing means for setting the normal gain of the repeater to the Value required for a particular normal attenuationof line sec- 10 tion, and. a second gain changing means adapted to be controlled in accordance with changes in attenuation of the'lfne, said second means operating to change the repeater gain according to a predetermined law regardless of the gain setting of the rst means, both of said gain changing means comprising variable capacity.

FRITHIOF B. ANDERSON. CHARLES O. MALLINCIQRODT. ALBERT L. STILLWELL. 

